1. Field of the Invention
This invention relates to a method of and an apparatus for image processing and, more particularly, to a method of and an apparatus for image processing, which permits various kinds of image processing.
2. Related Art
As prior art apparatus for color image reproduction, there are those using recording means of silver salt photographic type, electrophotographic type, thermal transfer type and so forth.
In electrophotographic and thermal transfer type recording means, three colors of yellow, magenta and cyan are used for color formation. In many recording means, a color material of black color is used to enhance the reproducibility of black. For example, it has been provided an appartus wherein if a color of original is judged to be a black, only a black color material is used.
In some prior art image processing apparatuses, spacial frequency characteristics of the original are judged, and the sharpness of recorded image is improved using edge emphasizing or like space filter means such that image areas with sharp image density changes are given sharper image density changes.
These apparatuses, however, have the following drawbacks.
Usually, for sharply recording edges of characters density changes of the original are extracted. Even in this case, erroneous judgement is prone in screen areas of the original, resulting in an unseemly screen reproduction image and in different color shades of edges of color characters from those in the original. Further, heretofore, a judgement level for judging character edges in images is fixed absolutely, making it difficult to select an adequate judgement level according to characteristics of a given image.
Further, heretofore character areas and intermedate image areas in a given image are judged as such, and these different areas are processed separately, i.e., the character areas are processed by pure binarization while the intermediate image areas are processed by providing a systematic dither process, for instance, thereby obtaining a satisfactory image.
Even this prior art method, however, still requires great improvement in connection with the function of discriminating character areas and intermediate image areas, and problems are encountered in connection with character areas and intermediate image areas, i.e., particular areas of image.
Further, as noted above in electrophotographic and thermal transfer type apparatuses three colors of yellow, magenta and cyan are used in superimposition on one another for color formation, and in many of these apparatuses color materials for black color are used to enhance the reproducibility of black.
In some of these apparatuses, colors of color originals is reproduced faithfully, and characters (particularly black characters) contained in color originals are reproduced sharply.
As an example, according to Japanese Patent Publication (Kokoku) No. 56-48869, the color of original is judged, and for a black area a sole color material for black color is used.
Further, in some apparatuses spacial frequency characteristics of original are determined, and the sharpness of recorded image is improved using edge emphasizing or like spacial filter means such that image areas with sharp image density changes are given sharper image density changes.
Further, in some of the recording means noted above a particular resolution of recording is generated psuedowise for graduation expression by using a systematic dither process or the like. Some of these apparatuses use such means as varying the dither matrix size by determining spacial frequency characteristics of the original or switching binarized image and dither image for clearly recording character edges or the like in the recorded image.
However, these apparatuses have the following drawbacks.
In an apparatus for monochromatic recording in black by judging original colors, black dots contained in a four-color screen print original produced by off-set printing or the like are emphasized, and an unseemly replica of image is obtained.
Further, in case where character edges are recorded sharply by extraction of original image density changes, screen areas of the original cause erroneous judgement to result in an unseemly replica of screen image. Further, color character edges cause different color shades from those in the original. Still further, there is no apparatus, which permits optimal separation of character areas, screen areas and photograph areas in case of full color copying and also in case of monochromatic copying.
There has been proposed a system, in which an original image is read out, edge areas containing characters and drawings and intermediate image areas containing photographs in the read-out image are separated from one another, and appropriate processes are provided for the respective separated areas (as disclosed in, for instance, "Nikkei Electronics", 1987, No. 425, p-p. 100-101.
In the prior art technique, however, when an original with intermediate images expressed by screen is read out for processing, the screen areas are judged as edge areas, and therefore undesired processing is provided.
Hitherto, there is known a technique of processing a read-out color image by spacial filtering using wellknown digital filter techniques. Further, there is an apparatus, in which the sharpness of the image quality (hereinafter referred to as sharpness) is changed continuously by continuously changing spacial frequency characteristics in spacial filter.
Meanwhile, there has been proposed a system, in which edge areas containing characters or the like and intermediate image areas containing photographs are separated from a read-out image, and appropriate processes are provided to the respective separated areas.
In the prior art apparatus, however, the same sharpness is provided for any of the developing colors of magenta (M), cyan (C), yellow (Y) and black (Br). Therefore, there are the following difficiencies.
When the sharpness is reduced in order to prevent noise in photograph areas or moire of screen, the sharpness of character areas is lost, and an image lacking sharpness is obtained.
Further, where the same sharpness is provided for character areas and flat areas such as photograph areas in an original at all times, in case of desiring an increased sharpness for increasing the sharpness of character areas, noise components in photograph areas are emphasized as well, or in case of desiring reduced sharpness to prevent noise in photograph areas or moire in screen the sharpness of character areas is lost.
Further, it is liable to judge intermediate tone areas expressed by screen in a portion of original to be edge areas or judge very thin characters as screen, and such "erroneous judgement" causes undesired processing of a portion of image.
Further, there is a problem in the prior art that with such areas as black characters a color blut of a certain color is generated around black areas due to such causes characteristics of processing circuits for processing individual color components of color image signal or lack of uniformity for individual colors of the sensitivity of a sensor section for converting a given image into a color image signal.
For this reason, there has been proposed a method of preventing the generation of color blurs (color bleeding) in colors round black character areas.
However, it has been confirmed by the inventors that black blur is generated around color characters in addition to color blurs around black characters.
More specifically, the black component is clearly recorded in areas corresponding to color character edges, thus degradating the quality of the reproduced image.